I've read it over and over again, that more bit-depth equates to more headroom. But I don't understand it; at least not in the practical sense.

As I've read/heard, the traditional method of recording 16-bit audio at 44.1k, one would try to record as loud as possible, but without clipping. This would produce the greatest signal-to-noise ratio. However, the risk would be that your performer could clip at any given moment because you are sacrificing headroom for greater signal-to-noise.

First of all, what "noise" does this scenario refer to?

Quantization noise?

Aliasing noise?

Internal hum/buzz from the recording equipment?

Background noise from your environment?

Other?

Secondly, how does recording louder produce a higher signal-to-noise ratio in each of these 5 types of noise? (Although #3 and #4 are obvious.)

In more modern times, I keep hearing about recording methods and recording devices that have huge amounts of headroom due to increased bit-depth. For example, when recording in Pro Tools, setting the parameters to 24-bit, 48k supposedly produces greater headroom. I also hear of portable recording devices that, as the box will say, has over 100dB of headroom. In the practical sense, I don't understand this because wouldn't you still want to record your signal as loud as possible while avoiding clipping? If so, then where's this "greater" headroom?

I think I have some answers to some of my own questions here, but I don't feel like I have a firm understanding of it, so I'd rather hear some answers from others.

When referring to recording headroom, it's always the noise floor of the recording medium that is being referred to. For digital, that's the quantization noise. With 16-bit, the quantization noise is usually higher than any analog self-noise. With 24-bit or greater the analog self-noise is higher than the quantization noise.

Aliasing has nothing to do with any noise floor or signal to noise ratio. Aliasing is where audio from above the Nyquist frequency is folded below it without adequate filtering.

With 24-bit, you shouldn't want to maximize the signal to noise ratio of each track. You'll find that the audio doesn't sound it's best when consistently pushed to the top of the A/D converter range. It also makes mixing more tedious as you have to deal with relative levels that can be all over the map.

16bit provides 96db of dynamic range. Your analog gear (and the analog section of your AD converter is likely 100db dynamic range (up to 120db for top shelf AD converters). So with 16 bit you want to print near peak to take advantage of the headroom.

24 bit provides 144db dynamic range (24db - 44db more than your analog gear. This means that you have up to 44db more dynamic range in digital than with your analog gear -so you could print with peaks at -20db in digital and the noise of your analog gear would still be greater than the noise floor of digital.

To make things easy, setting your converters to -18dbfs is a safe bet (and is the default for many interfaces like the Lynx Aurora). It means 0vu in analog comes into digital at -18, so even if you have peaks at +10db in analog you still have 8db of digital headroom.